Detailed kinetic modeling of NOx storage and reduction with hydrogen as the reducing agent and in the presence of CO2 and H2O over a Pt/Ba/Al catalyst

Abstract A detailed kinetic model of NO x storage and reduction, in the presence of H 2 O and CO 2 , with hydrogen as the reducing agent was developed and validated in this study. The mechanism was derived from flow reactor experiments conducted at 200–400 °C over a Pt/Ba/Al monolith sample. The detailed kinetic model is divided into four sub-models: (i) NO oxidation over Pt, (ii) NO x storage, (iii) NO x reduction over Pt, and (iv) NO x regeneration. The sub-model for NO x storage is based on our earlier work and is further developed in this study to include high concentrations of CO 2 and H 2 O in the feed and also low temperature storage. In the model NO x is allowed to be stored on two different types of storage sites: BaCO 3 and a second storage site denoted S 3 . Based on experimental results many studies suggests multiple storage sites one these catalysts, and there are different explanations (i) alumina and barium sites (ii) bulk and surface barium, (iii) barium close and far from the noble metal, etc. The disproportionation route, where NO 2 is stored over BaCO 3 , is included in the NO x storage model. To account for the storage occurring at lower temperatures NO can be stored over both BaCO 3 and S 3 in the presence of O 2 . NO adsorbed over S 3 can be further oxidized to NO 2 by reacting with oxygen on neighboring Pt sites. The second storage site (S 3 ) is important in order to explain NO x storage at low temperatures and the disproportionation reaction is essential to describe the storage at high temperatures. The NO x reduction sub-model used here was developed earlier over Pt/Si. Additional to the reduction of NO x into N 2 , it describes the formation of NH 3 over Pt. In the sub-model describing the regeneration of NO x , adsorbed NO x species react with hydrogen adsorbed on Pt sites. Ammonia oxidation over Pt and reactions between surface species of barium and NH 3 according to ammonia selective catalytic reduction (SCR) chemistry are also incorporated in the regeneration sub-model. The full model can describe the complete uptake of NO x in the beginning of the lean period, the NO x breakthrough, and the slow NO x storage in the end of the lean period very well as well as the following release and reduction. It can also predict the gradual decrease in the storage capacity occurring in lean/rich cycling experiments. Furthermore, the ammonia formation predicted by the model fits well with experimental data. The model was validated with short lean (60 s) and rich (15 s) cycles which were not included in the model development. The model could predict these experiments well for all three temperatures (200, 300 and 400 °C).

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